Hi Guys, first let me give you some detail of what Audi A6 I have.
I got a 1998 Audi A6 Sedan Quattro. 2.8L 30V V6 Engine. With original Cassatte Tape Player with non-bose system. I only have speakers/tweeter on the both front and rear doors, but I don't think the rear deck.

Recently, I am sick of the Cassatte Player, so I changed to a Sony MP3/CD player. MD8100X. I rewired the cables in the console to my sony player. I also replaced the rear both door speaker with Sony 4 way speakers. but now, when I play, not all the time, but I believe when the sound is different for right and left (stereo), rear speakers make like (plar..plar...sound) sound like blown speakers. but they are brand new. Also even with my old tape player originally. rear speaker is making that noise already. It only happen when I turn the volume to a certain level.. not too loud..but like volume 3 out of 10. Funniest thing is, when I change the balance on my CD player to only left or right completely, it trigger this loud noise eaiser (like 2 out of 10 volume) and both speakers at the rear has the noise(including tweeter also) even I only put the balance to only right or left. Any help?
I think maybe is the amplifer is bad? I didn't rewire the audio cable from HU to the rear speakers, I try not to go into too much trouble. if it is the amp, I rather just remove it. But I cannot find it. not at the rear door. I looked at every spot of both rear door. And it's not on the stock speaker. Any one know where is the amp? and what caused the noise?

Also, when there is no plar plar sound, the sound is perfect. .. not static, no nosie.. the bass is good, and everything.. just somehow, it will have those PLAR...PLAR.. sound. very weird!

Sorry for the really long post!
Thanks all. appreciated the help!
Hope to get this fixed soon!
Gil

I think I have the same problem with my Bose system ('99 A4 1.8T). The dealership told my dad that the speaker was blown and charged him something rediculous to replace it ($240?). Less than a month later, the sound was back. In my car, it only happens at certain times. If the speaker was blown, it would consistently sound pretty much the same. I think that the rear deck is poorly designed and it is actually causing the sound. I don't know how to fix it, but I'm more passionate about music than my dad... so my first priority is to fix this.

Let me know if you find anything, and I'll do the same if I figure it out first.

It might be resonance. You can buy a cheap spray which would cost about $5 or so, which would coat the metal shelf, and assist in fighting the resonance. Additionally, you could lay dynamat pieces over the metal shelf, cut to size, keeping the vents open. And on top of that, you can then stuff polyfiber sound deadening material between the wooden shelf and metal shelf. You can buy all these things on sites like crutchfield. If you don't want to buy actual sound deadening fill, you can chop up a cheap polyester pillow, as it will provide some deadening.

If your problems are in the rear shelf, and this doesn't fix it, it's prolly something else causing the problem.

I had my rear deck Bose speakers blow on me. It really sucks....$160 a pop, not including installation. [:@]

My update to this thread is this. i pulled out the rear shelf, turned on the radio. weird buzzing was from the speaekrs and not resonance. woofers don't appear to be blown, and it doesn't sound like the voice coil, almost like the amp is clipping wayy hard. my ultimate decision was to just unplug the system. sounds fine w/o them hooked up. i'm tempted to throw in a 10" sub to add more lows again, but the stereo system sounds incredibly similar to my dad's e36 m3 system. Not too shabby at all.